Tube Power Amp Reviews

I knew nothing of Ypsilon when I first saw its products in a room at an overseas audio show. Even though the speakers in this system were complete unknowns, I was convinced that it was the electronics that were responsible for the magical balance of what I was hearing. That was confirmed when I reviewed the VPS-100 phono preamplifier in August 2009 and PST-100 Mk.II preamplifier in July 2011.

Greek austerity ends at the factory door of Ypsilon Electronics. The luxury components designed and manufactured within are innovative, high-performance, visually elegant, and expensive. They're aimed at audio enthusiasts, mostly outside Greece, who can afford to indulge themselves. Obviously, no Greek would object—the nation's economy, and those employed by Ypsilon, can only benefit from this small company's success, however minor the contribution.

Ypsilon's Aelius monoblocks cost $36,000/pair. That's a lot of money, but compared to some really expensive gear, including the Wilson Alexandria XLF speakers I reviewed in January ($200,000/pair) and Ypsilon's own SET 100 Ultimate amplifiers ($125,000/pair), the Aelius may be, for some, as Ypsilon's slogan suggests, "untouchable . . . but not unreachable."

Tube-rectified tube input, MOSFET output
The Aelius is moderately large and blocky; it measures 16.6" square by 9" tall and weighs 99 lbs. It's specified as outputting 200W into 8 ohms, 380W into 4 ohms, or 500W into 2 ohms, with the first 60W in pure class-A. Its tastefully understated satiny finish, sculpted front accent, and blue LED slit match the looks of the rest of the Ypsilon line.

Electrical engineer Demetris Backlavas has designed a circuit for the Aelius that has only two gain stages and almost no passive components in the signal path. The single-ended class-A tubed input stage uses a single C3g pentode tube (rectification for this stage is supplied by another tube) transformer-coupled to the push-pull output stage of six matched pairs of N-channel polarity MOSFETs. There are no source resistors in the circuit.

An interstage transformer is unusual in a solid-state or hybrid amplifier; one is used here as a "perfect" phase splitter for the Aelius's push-pull operation, and as a step-down transformer to greatly lower the input tube's impedance to drive the capacitance of the output MOSFETs. Though the phase is split and the amp is push-pull, it's really two single-ended amplifiers with six transistors driving the plus terminal and six driving the minus terminal—a very unusual circuit.

I visited Ypsilon while in Athens a few years ago. One afternoon, using an amplifier he was designing, Backlavas demonstrated the significant sonic differences produced by swapping out various capacitors, transformer core materials, and other components. The guy is steeped in theory but, ultimately, guided by his ears. Like a great chef, he knows how to obtain and mix the best ingredients to produce a sublime dish.

Backlavas points out that, without the transformer, an additional one or two active stages would have been needed to split the phase and lower the impedance and that those additional stages would reduce the amp's notable transparency and signal purity. Of course, transformers have their own problems, and building one with a sufficiently wide bandwidth (in this case, 10Hz–70kHz) is both difficult and necessary: the transformer's bandwidth defines the amplifier's bandwidth.

Backlavas says that while the Aelius's circuit is in some ways similar to the Circlotron configuration used in some output-transformerless (OTL) tube amps, it is not a true Circlotron. Unlike the Circlotron's unity-gain output, the Aelius's output is greater than unity gain. The result, Backlavas claims, is an amplifier with the sonic purity of a single-ended design, with push-pull power sufficient to drive virtually any loudspeaker.

Easy Connections
The Aelius's flat rear panel made all connections easy. Each of the "pure copper and gold-plated" speaker terminals has a large, round, screw-in knob of frosted plastic. These knobs are big enough to produce enough torque for a secure fit with even the stiffest cable when tightened by hand, and there's enough space between them to accept spades of any size, as well as banana plugs. Don't diminish the importance of this aspect of amplifier design—there's nothing more annoying than stupidly designed and/or placed terminals that seem to have been created by people who have never actually connected a speaker cable to an amplifier. Pay attention to this when you shop.

There are both single-ended RCA and balanced XLR inputs, chosen with an adjacent switch. Also on the rear panel is a handy ground lift switch for thwarting ground loops; this disconnects the ground circuit from the chassis ground. As with all Ypsilon products I've reviewed, the On/Off switch is on the rear.

As Supplied
Ypsilon specifies for the Aelius new old stock (NOS) of the military-grade Siemens C3g tube, which has eight pins, a metal sleeve, and a minimum lifespan of 10,000 hours. Unbeknownst to me, my review samples had been fitted with a Russian-made Electro-Harmonix 6C45PiEH tube, each with nine gold pins, that had been soldered into circular eight-pin adapter plates. (More below about why the swap was made.) I didn't discover this until after a month or so of listening, during which time I was also reviewing the Dan D'Agostino Momentum monoblocks (reviewed in February 2013), and using as references darTZeel's big NHB-458 monoblocks (August 2012).

With the 6C45PiEH tube, the Aelius produced a warm, voluptuous, somewhat darkly "tubey" sound that gave little hint of its solid-state MOSFET output (though MOSFETs are reputed to have a warmer, softer sound than bipolar devices). Yet despite the voluminous soundstage and generous bloom, instrumental attacks were precise and transients were cleanly delineated through much of the audioband, though the bass was less than taut, and not as nimble and as well controlled as I like it—and as I know the Wilson Alexandria XLFs are capable of producing.

Driving the very sensitive XLFs, the Aelius was probably running in class-A all the time, even when producing high (sometimes ridiculously high) SPLs in my moderately sized room. Considering the Aelius's class-A operation, zero feedback, and two-stage simplicity, it produced less transparency, and less of a direct, "straight-through" sound, than I'd expected. Nor, despite the use of tubes, did it produce the uncanny tonal neutrality I've come to expect from Ypsilon electronics.

My family visits during the holidays, and both of my sisters and my brother-in-law usually indulge me by spending 10 minutes or so listening to "what's going on down there" in my basement listening room. This time, the three of them sat there for well over an hour, and had to be prodded into going back upstairs. They sat through a side of Mel Tormé and Friends: Recorded Live at Marty's, New York City (2 LPs, Finesse W2X37484), and all of side 2 of the Beatles' Abbey Road. Unprecedented! What kept them sitting? The speakers, of course, but as driven by the Aeliuses, the width and depth of the Wilsons' soundstage was unusually enormous and enveloping—overwhelming, actually, and almost in our laps. Add a mesmerizing ease of sound that produced both reasonably good resolution of detail and billowy amounts of air and spaciousness, and it made for a very "wow" experience for all of us. The Mel Tormé record, in particular, was as "you are there" live as I've ever heard it.

Still, to my more experienced and critical ear the sound was overripe, somewhat diffuse, and tended toward softness, both at the very bottom and in the lower midrange, despite the latter's most attractive harmonic richness and the overall sound's uncanny textural verisimilitude.

The Aeliuses reproduced full-bodied, woody-sounding pianos from good recordings of solo pianos, but the attack cheated on the soft, diffuse, romantic side—too soft to correctly reproduce the attack of either startling and appropriately hard fortissimos or delicate yet well-focused pianissimos, not to mention all the dynamic gradations in between. Of course, the sound was always pleasing, but in the way that some audiophiles tend to romanticize how live music actually sounds.

It took me decades to acquire a system capable of cleanly and accurately delineating the honky-tonk piano of Nicky Hopkins (not Ian Stewart) in "Rocks Off," from the Rolling Stones' raucous Exile on Main Street (LP)—not to mention a system capable of separating out most of the parts from what most critics (with crappy systems) declared, in the early '70s, was "total sonic murk"—not that the proper playback of that particular album was my long-term goal!

Clearly hearing instruments and voices heretofore buried in the mix has always been among the more revelatory experiences of a worthwhile system upgrade, though it's not as wondrous as when a new component increases the listener's understanding of the music. The softness of that piano part clearly demonstrated the general softness of the Ypsilons' sound, particularly in the lower mids.

Switching to the admittedly more expensive D'Agostino Momentums ($55,000/pair), or to the hideously more expensive darTZeel NHB-458s ($150,000/pair), revealed—at least in my system—not only the Aelius's transient softness, but a dab of Vaseline on its sonic lens. This made everything sound "good," but robbed the greatness from recordings that truly were. Transparency, in particular, suffered.

Normally, I would have simply reviewed the amps as delivered, but given that these samples strayed from the preternaturally neutral, natural sound produced by every other Ypsilon product I've reviewed or heard at audio shows, I thought it best to contact the designer. Plus, last year, when a pair of preproduction Aeliuses were on the East Coast for some reason, Backlavas had asked if I'd like to hear them. I'd said I did, but that I wouldn't comment on what I heard or didn't hear.

The review samples did not sound at all like the preproduction pair. As I recall, Backlavas did tell me that for his first production run he was changing the material of the Aelius's transformer core, among other things, which would affect the sound, but the differences in sound between the two pairs of amps were so big that I called him.

After inquiring about the overall softness, and asking if that was what I was supposed to be hearing, I was told to expect another pair of input tubes. When the box arrived, I wasn't surprised to find in it a pair of C3g tubes, as specified in the instruction manual. But when I removed the amplifiers' ornately machined top plates, I was surprised to find, inside each, a 6C45PiEH instead of a C3g.

At the 2013 Consumer Electronics Show, I asked Backlavas what was going on. It turned out to be a combination of designer second-guessing and bureaucratic bullshit. Since 2006, the European Union's RoHS regulations (which mandate that solder be lead free but not CRTs—a much greater source of lead contamination, you can be sure) require documentation proving that no part of a new electronic component contains hazardous metals. This essentially outlawed the use of NOS vacuum tubes, of which the C3g is but one of many models, because no documentation exists that can prove their compliance with the regulations. While many high-performance audio manufacturers use NOS tubes anyway and ship to EU countries, Ypsilon was concerned about this and sought an alternative, currently manufactured tube. The 6C45PiEH proved a good choice that produced "interesting" results, Backlavas told me, adding that the tube "has midrange energy and body but it's darker and warmer."